Inactivation of the olfactory amygdala prevents the endocrine response to male odour in anoestrus ewes

Perspective: Re-defining “Pheromone” in a Mammalian Context to Encompass Seminal Fluid

The classical view of “pheromone”—an air-borne chemical signal—is challenged by the camelids in which ovulation is triggered by ß-nerve growth factor carried in seminal plasma, effectively extending the pheromone concept to a new medium. We propose further extension of “pheromone” to include a separate class of seminal fluid molecules that acts on the female reproductive tract to enhance the prospect of pregnancy. These molecules include transforming growth factor-ß, 19-OH prostaglandins, various ligands of Toll-like receptor-4 (TLR4), and cyclic ADP ribose hydrolase (CD38). They modulate the immune response to “foreign” male-derived histocompatibility antigens on both sperm and the conceptus, determine pre-implantation embryo development, and then promote implantation by increasing uterine receptivity to the embryo. The relative abundance of these immunological molecules in seminal plasma determines the strength and quality of the immune tolerance that is generated in the female. This phenomenon has profound implications in reproductive biology because it provides a pathway, independent of the fertilizing sperm, by which paternal factors can influence the likelihood of reproductive success, as well as the phenotype and health status of offspring. Moreover, the female actively participates in this exchange—information in seminal fluid is subject to “cryptic female choice,” a process by which females interrogate the reproductive fitness of prospective mates and invest reproductive resources accordingly. These processes participate in driving the evolution of male accessory glands, ensuring optimal female reproductive investment and maximal progeny fitness. An expanded pheromone concept will avoid a constraint in our understanding of mammalian reproductive biology.

Patterns of preoptic-hypothalamic neuronal activation and LH secretion in female sheep following the introduction and withdrawal of novel males

The neuroendocrine response of female sheep to a novel male involves neural activation in the hypothalamus. However, if males are removed, the gonadotrophic signal declines, so the neural activity is likely to change. We examined Fos-immunoreactive (IR) cells in hypothalamic tissues from seasonally anovulatory female sheep exposed to males for 2 or 6h, or for 2h followed by 4h isolation from males. Control females were killed in the absence of male exposure. Male introduction increased LH secretion in all females; male removal was associated with a reduction only in mean and basal LH concentrations. Females exposed to males for 2h had more Fos-IR cells in the arcuate nucleus (ARC), ventromedial nucleus of the hypothalamus (VMH) and organum vasculosum of the lamina terminalis (OVLT) than control females. Fos-IR cells in the preoptic area (POA) were only greater than in control females after 6h exposure to a male. Removal of males decreased the number of Fos-IR cells in the ARC, VMH and OVLT, but not in the POA. Thus, hypothalamic neural activation and LH secretion in female sheep are stimulated by males and decline after male removal. However, activation in the POA persists after removal and may explain the incomplete decline in the LH response.

The Two Populations of Kisspeptin Neurons Are Involved in the Ram-Induced LH Pulsatile Secretion and LH Surge in Anestrous Ewes

Exposure to a ram during spring stimulates luteinizing hormone (LH) secretion and can induce ovulation in sexually quiescent ewes ("ram effect"). Kisspeptin (Kiss) present in the arcuate nucleus (ARC) and the preoptic area (POA) is a potent stimulators of LH secretion. Our aim was to investigate whether Kiss neurons mediate the increase in LH secretion during the ram effect. With double immunofluorescent detection, we identified Kiss neurons (Kiss IR) activated (Fos IR) by exposure to a ram for 2 hours (M2) or 12 hours (M12) or to ewes for 2 hours (C). The density of cells Kiss + Fos IR and the proportion of Kiss IR cells that were also Fos IR cells were higher in M2 and M12 than in C in ARC (P < 0.002) and POA (P < 0.02). In ARC, these parameters were also higher in M12 than in M2 (P < 0.02 and P < 0.05). Kiss antagonist (P234 10-6M) administered by retrodialysis in POA for 3 hours at the time of introduction of the ram reduced the amplitude of the male-induced increase in LH concentration compared with solvent (P < 0.02). In ARC, P234 had a more limited effect (P < 0.038 1 hour after P234) but pulse frequency increased less than after solvent (P = 0.07). In contrast, Kiss antagonist (P271 10-4M) infused in ARC but not POA 6 to 18 hours after introduction of the ram prevented the LH surge in the ewe (0/6 vs 4/5 and 4/6 in C). These results suggest that both populations of Kiss neurons are involved in the ram-induced pulsatile LH secretion and in the LH surge.

The "ram effect": new insights into neural modulation of the gonadotropic axis by male odors and socio-sexual interactions

Reproduction in mammals is controlled by the hypothalamo-pituitary-gonadal (HPG) axis under the influence of external and internal factors such as photoperiod, stress, nutrition, and social interactions. Sheep are seasonal breeders and stop mating when day length is increasing (anestrus). However, interactions with a sexually active ram during this period can override the steroid negative feedback responsible for the anoestrus state, stimulate luteinizing hormone (LH) secretion and eventually reinstate cyclicity. This is known as the “ram effect” and research into the mechanisms underlying it is shedding new light on HPG axis regulation. The first step in the ram effect is increased LH pulsatile secretion in anestrus ewes exposed to a sexually active male or only to its fleece, the latter finding indicating a “pheromone-like” effect. Estradiol secretion increases in all ewes and this eventually induces a LH surge and ovulation, just as during the breeding season. An exception is a minority of ewes that exhibit a precocious LH surge (within 4 h) with no prior increase in estradiol. The main olfactory system and the cortical nucleus of the amygdala are critical brain structures in mediating the ram effect since it is blocked by their inactivation. Sexual experience is also important since activation (increased c-fos expression) in these and other regions is greatly reduced in sexually naïve ewes. In adult ewes kisspeptin neurons in both arcuate and preoptic regions and some preoptic GnRH neurons are activated 2 h after exposure to a ram. Exposure to rams also activates noradrenergic neurons in the locus coeruleus and A1 nucleus and increased noradrenalin release occurs in the posterior preoptic area. Pharmacological modulation of this system modifies LH secretion in response to the male or his odor. Together these results show that the ram effect can be a fruitful model to promote both a better understanding of the neural and hormonal regulation of the HPG axis in general and also the specific mechanisms by which male cues can overcome negative steroid feedback and trigger LH release and ovulatory cycles.

Chemosignals and hormones in the neural control of mammalian sexual behavior

Males and females of most mammalian species depend on chemosignals to find, attract and evaluate mates and, in most cases, these appetitive sexual behaviors are strongly modulated by activational and organizational effects of sex steroids. The neural circuit underlying chemosensory-mediated pre- and peri-copulatory behavior involves the medial amygdala (MA), the bed nucleus of the stria terminalis (BNST), medial preoptic area (MPOA) and ventromedial hypothalamus (VMH), each area being subdivided into interconnected chemoreceptive and hormone-sensitive zones. For males, MA-BNST connections mediate chemoinvestigation whereas the MA-MPOA pathway regulates copulatory initiation. For females, MA-MPOA/BNST connections also control aspects of precopulatory behavior whereas MA-VMH projections control both precopulatory and copulatory behavior. Significant gaps in understanding remain, including the role of VMH in male behavior and MPOA in female appetitive behavior, the function of cortical amygdala, the underlying chemical architecture of this circuit and sex differences in hormonal and neurochemical regulation of precopulatory behavior.

Kisspeptin: a new neuronal target of primer pheromones in the control of reproductive function in mammals

Pheromones are known to trigger either short-term behavioral responses, usually referred to as "releaser effects", or more long-term physiological changes, known as "primer effects", which especially affect reproductive function at the level of the gonadotrope axis. The precise mechanisms through which pheromones interact with the gonadotrope axis in the hypothalamus is not fully known. We propose that the neuropeptide Kisspeptin, could be a specific target of primer pheromones, allowing these pheromones to modulate the gonadotrope axis and GnRH activity. This emerging hypothesis is discussed in the context of puberty acceleration in female mice and the male effect in female ungulates (sheep or goat). These examples have been chosen to illustrate the diversity of the reproductive contexts in mammals and potential mechanisms affected by primer effects at the level of the gonadotrope axis.

Chemosignals, hormones and mammalian reproduction

Many mammalian species use chemosignals to coordinate reproduction by altering the physiology and behavior of both sexes. Chemosignals prime reproductive physiology so that individuals become sexually mature and active at times when mating is most probable and suppress it when it is not. Once in reproductive condition, odors produced and deposited by both males and females are used to find and select individuals for mating. The production, dissemination and appropriate responses to these cues are modulated heavily by organizational and activational effects of gonadal sex steroids and thereby intrinsically link chemical communication to the broader reproductive context. Many compounds have been identified as "pheromones" but very few have met the expectations of that term: a unitary, species-typical substance that is both necessary and sufficient for an experience-independent behavioral or physiological response. In contrast, most responses to chemosignals are dependent or heavily modulated by experience, either in adulthood or during development. Mechanistically, chemosignals are perceived by both main and accessory (vomeronasal) olfactory systems with the importance of each system tied strongly to the nature of the stimulus rather than to the response. In the central nervous system, the vast majority of responses to chemosignals are mediated by cortical and medial amygdala connections with hypothalamic and other forebrain structures. Despite the importance of chemosignals in mammals, many details of chemical communication differ even among closely related species and defy clear categorization. Although generating much research and public interest, strong evidence for the existence of a robust chemical communication among humans is lacking.

The main but not the accessory olfactory system is involved in the processing of socially relevant chemosignals in ungulates

Ungulates like sheep and goats have, like many other mammalian species, two complementary olfactory systems. The relative role played by these two systems has long been of interest regarding the sensory control of social behavior. The study of ungulate social behavior could represent a complimentary alternative to rodent studies because they live in a more natural environment and their social behaviors depend heavily on olfaction. In addition, the relative size of the main olfactory bulb (MOB) [in comparison to the accessory olfactory bulb (AOB)] is more developed than in many other lissencephalic species like rodents. In this review, we present data showing a clear involvement of the main olfactory system in two well-characterized social situations under olfactory control in ungulates, namely maternal behavior and offspring recognition at birth and the reactivation of the gonadotropic axis of females exposed to males during the anestrous season. In conclusion, we discuss the apparent discrepancy between the absence of evidence for a role of the vomeronasal system in ungulate social behavior and the existence of a developed accessory olfactory system in these species.

Sociosexual stimuli and gonadotropin-releasing hormone/luteinizing hormone secretion in sheep and goats

Sociosexual stimuli have a profound effect on the physiology of all species. Sheep and goats provide an ideal model to study the impact of sociosexual stimuli on the hypothalamic-pituitary-gonadal axis because we can use the robust changes in the pulsatile secretion of luteinizing hormone as a bioassay of gonadotropin-releasing hormone secretion. We can also correlate these changes with neural activity using the immediate early gene c-fos and in real time using changes in electrical activity in the mediobasal hypothalamus of female goats. In this review, we will update our current understanding of the proven and potential mechanisms and mode of action of the male effect in sheep and goats and then briefly compare our understanding of sociosexual stimuli in ungulate species with the "traditional" definition of a pheromone.

Effect of Blockage of the Ducts of the Vomeronasal Organ on LH Plasma Levels during the "Whitten Effect" in Does

Eighteen mature, nonpregnant, and indigenous South African does were randomly divided into two groups to test if their vomeronasal organs exert an influence on LH plasma levels during a Whitten effect experimental trial. Does in the treatment (VNO ablated) group had their vomeronasal organs rendered nonfunctional by cauterization of the nasoincisive duct under surgical anesthesia. Does in the control group had their nasal civities irrigated with physiological saline under surgical anesthesia. All does were synchronized into oestrus and introduced to bucks one day prior to their expected second oestrus cycle. Successful matings were recorded. Timely blood samples were collected during each of the five days before and five days after buck introduction. Blood plasma concentrations of estradiol and LH were determined by radioimmunoassay. Analysis of variance between groups demonstrated that the does in the VNO ablated group did not demonstrate any interest in mating, did not become pregnant, and did not demonstrate the primary increase in tonic plasma levels of LH that is necessary for ovulation to occur. By contrast, all of the does in the control group demonstrated successful matings, became pregnant, and demonstrated typical primary tonic level increases and preovulation surges in LH. Thus, it was concluded that the vomeronasal organ modulates the primary increase in tonic levels of LH and thus influences ovulation that occurs during the Whitten effect in South African indigenous does.

The 'male effect' in sheep and goats--revisiting the dogmas

Male-induced ovulation in sheep and goats (the 'male effect'), documented during the period 1940-1960, has long been shrouded in preconceptions concerning how, when and why it worked. These preconceptions became dogmas but recent research is challenging them so, in this review, we have re-visited some major physiological (breed seasonality; characteristics of the response; the nature of the male stimuli) and physical factors (duration of male presence; isolation from male stimuli) that affect the phenomenon. We reject the dogma that ewes must be isolated from males and conclude that male 'novelty' is more important than isolation per se. Similarly, we reject the perception that the neuroendocrine component of the male effect is restricted to anovulatory females. Finally, we re-assess the relative importance of olfactory and non-olfactory signals, and develop a perspective on the way male-induced ovulation fits with preconceptions about pheromonal processes in mammals. Overall, our understanding of the male effect has evolved significantly and it is time to modify or reject our dogmas so this field of research can advance. We can now ask new questions regarding the application of the male effect in industry and develop research so we can fully understand this biological phenomenon.

Role of the olfactory systems and importance of learning in the ewes' response to rams or their odors

In sheep, exposure of seasonally anestrous females to the male or its fleece results in activation of luteinizing hormone (LH) secretion and synchronized ovulation. The study of the neural pathways involved in this phenomenon, commonly named "male effect", show that the main olfactory system plays a critical role in the detection and the integration of the male odor. The accessory olfactory system participates in the perception of the ram odor but does not seem necessary for the endocrine response. According to the hypothesis that the neuroanatomical differences between the two olfactory systems could be associated with different functional roles, we investigated the importance of sexual experience and learning processes in the male effect. Our results showed that female responses depend on previous sexual experience. We also demonstrated that the LH response to male odor could result from an associative learning process. The aim of the present report was to summarize our current knowledge concerning the "male effect" and in particular to clarify the role of sexual experience and learning in the processes involved in this effect.

Neural pathways involved in the endocrine response of anestrous ewes to the male or its odor

During the non-breeding season, anestrous ewes do not experience ovarian cycles but exposure to a ram or its odor results in the activation of the luteinizing hormone secretion leading to ovulation. The aim of our work was to identify the neural pathways involved in this phenomenon. Using Fos immunocytochemistry, we examined the brain areas activated by the male or its fleece, in comparison with ewes exposed to the female fleece or the testing room (control group). In comparison with the control group, the male or its odor significantly increases Fos neuronal expression in the main and accessory olfactory bulbs, anterior olfactory nucleus, cortical and basal amygdala, dentate gyrus, ventromedial nucleus of the hypothalamus, piriform and orbitofrontal cortices. The main olfactory bulb, the cortical amygdala and the dentate gyrus are specifically more activated by the male odor than the female odor. Using a procedure of double labeling for Fos and gonadotropin-releasing hormone, we also compared the number of gonadotropin-releasing hormone neurons activated in the four groups of females. The male or its odor significantly increases the number and the proportion of gonadotropin-releasing hormone cells expressing Fos-immunoreactivity in the preoptic area and the organum vasculosum of the lamina terminalis, whereas no such induction of Fos-immunoreactivity was found in gonadotropin-releasing hormone neurons of ewes exposed to the female odor or the testing room. These findings emphasize the role of the main olfactory system in the detection and the integration of the ram odor, and also suggest the participation of the accessory olfactory system. Numerous structures widely distributed seem involved in the processing of the male olfactory cue to reach the gonadotropin-releasing hormone neurons.

Cortical and medial amygdala are both involved in the formation of olfactory offspring memory in sheep

Ewes form a selective olfactory memory for their lambs after 2 h of mother-young interaction following parturition. Once this recognition is established, ewes will subsequently reject any strange lamb approaching the udder (i.e. maternal selectivity). The present study tested the functional contribution of different amygdala nuclei to lamb olfactory memory formation. Using the anaesthetic lidocaine, cortical, medial or basolateral nuclei of the amygdala were transiently inactivated during lamb odour memory formation. Reversible inactivation of either cortical or medial amygdala during the first 8 h postpartum impaired lamb olfactory recognition, whereas inactivation of the basolateral nucleus or infusion of artificial cerebrospinal fluid did not. Control experiments indicate that inactivation of the cortical and medial nuclei of the amygdala specifically disrupt memory formation rather than olfactory perception or memory retrieval. These findings show that both nuclei of the amygdala are required for the formation of a lamb olfactory memory and suggest functional interaction between these two nuclei.

The "male effect" in sheep and goats: a review of the respective roles of the two olfactory systems

In sheep and goats, exposure of seasonally anestrous females to sexually active males results in activation of luteinizing hormone (LH) secretion and synchronized ovulation. This phenomenon is named "the male effect" and seems to constitute a major factor in the control of reproductive events. This effect depends mostly on olfactory cues and is largely mimicked by exposure to male fleece only. In sheep, preventing the vomeronasal organ (VNO) from functioning does not affect the female responses to male odor suggesting that, unlike in rodents, the accessory olfactory system does not play the major role in the perception of this pheromonal cue. Female responses also seem to depend on previous experience, an effect that is not common for pheromones and renders this model of special interest. The aim of the present report is to summarize our current knowledge concerning the "male effect" and in particular to clarify the respective roles of the two olfactory systems in the processes involved in this effect.

Mapping the Neural Substrates Involved in Maternal Responsiveness and Lamb Olfactory Memory in Parturient Ewes Using Fos Imaging

In sheep, recognition of the familiar lamb by the mother depends on the learning of its olfactory signature after parturition. The authors quantified Fos changes in order to identify brain regions activated during lamb odor memory formation. Brain activation was compared with those measured in anosmic ewes displaying maternal behavior but not individual lamb recognition. In intact ewes, parturition induced significant increase in Fos expression in olfactory cortical regions and in cortical amygdala, whereas in anosmic mothers, Fos expression was very low. In contrast, no difference was observed between intact and anosmic ewes in hypothalamic areas and medial amygdala, suggesting a differentiation between the neural network controlling maternal responsiveness and that involved in olfactory lamb memory.